wgrep doesn't work with Emacs 26 default grep args

[Joe Shaw]

From the Emacs 26 release notes:

*** Grep commands will now use GNU grep's '--null' option if
available, which allows distinguishing the filename from contents if
they contain colons. This can be controlled by the new custom option
'grep-use-null-filename-separator'.

This breaks wgrep mode, as editing lines prints only "Text is read-only". For more background, see https://groups.google.com/d/topic/gnu.emacs.help/uu6ZQZGQ9FA/discussion

If you remove the --null option from the grep command-line things work.